Abstract:Based on the observed station precipitation data over China in winter,the monthly Hadley Center Global Sea Ice and Sea Surface Temperature(HadISST) and the monthly reanalysis data provided by National Centers for Environmental Prediction-National Center for Atmospheric Research(NCEP/NCAR) from 1951 to 2010,the relationship between the western Pacific warm pool(WPWP) and winter precipitation over South China(SC) has been studied.The results show that the correlation between WPWP and winter precipitation over SC has obvious interdecadal transition in 1972.Before the 1970s,the negative correlation between them is weak and has not passed significance test.But the correlation between them has reached-0.5——0.6 after the 1970s.By the regression analysis,it is found that,before the 1970s,the effect of WPWP on water vapor and dynamic conditions that affect winter rainfall over SC is not significant,so its relation with winter rainfall over SC is not obvious.After the 1970s,when WPWP SST is warmer,an obvious cyclonic circulation has been aroused in the Philippine Sea area.The northerly wind over the northwest side of the cyclonic circulation is not favor of water vapor from South China Sea transported to SC,so the water vapor divergence over SC increases.The convection and perturbation over the southern branch trough region weaken significantly and the perturbation can not spread to the downstream region,which causes the weakened perturbation over SC.In addition,the warming of WPWP leads to remarkable increase of meridional circulation in the Northern Hemisphere,and the sinking branch is located in SC region,which strengthens the sinking movement over SC,so it results in the decreased rainfall over SC in winter,and vice versa.

Abstract:Based on the RMM(Real-time Multivariate Madden-Julian oscillation) index from Australia Meteorological Bureau,the tropical atmospheric intraseasonal oscillation is divided into eight phases.The variations of typhoon genesis frequency and location over the northwestern Pacific during the eight phases are statistically analyzed.It is found that more(less) typhoons generate during the active phases 5,6 and 7(the inactive phases 1,2 and 3) over the northwestern Pacific.The box difference index(BDI) is further used to investigate the differences in environmental factors between the active and inactive phases.Results show that,compared to those during the inactive phases 1,2 and 3,the stronger low level convergence and upper level divergence outflows,the higher relative humidity in middle troposphere,as well as wider areas with small vertical shear can be found during the active phases 5,6 and 7.Moreover,the most significant difference in large scale environment field between the inactive and active phases is dynamical factor,especially low level convergence field.

Abstract:Based on the monthly mean global data from CMIP5(Coupled Model Intercomparison Project Phase 5),this paper studies relationship between sea surface temperature(SST) variation and precipitation variation over tropical Pacific and Indian Ocean along with its possible causes from the point of seasonal variation.The ensemble mean results of 20 models show that,under the background of global warming,annual spatial distributions of SST and precipitation variations over tropical Pacific Ocean accord with the "warmer-get-wetter" mechanism.However,the differences between seasonal mean and annual mean are obvious.Both zonal and meridional positional deviations between the local maximum enhanced SST and increased precipitation are detected in winter and spring,but only opposite meridional deviation is detected in summer and autumn.Meanwhile,the similar positional deviation between SST and precipitation exits over tropical Indian Ocean in winter and spring as well.Moreover,seasonal changes of precipitation over two tropical oceans accord with both the "warmer-get-wetter" mechanism and the "wet-get-wetter" mechanism.

Abstract:Using the global monthly mean sea surface temperature(SST) data,the ONI(Oceanic Nino Index) series,and the monthly rainfall of 160 stations in China,El Nino events are classified in several ways,and the influences of all kinds of El Nino events on East Asia circulation and the monthly and seasonal precipitation in following summer in eastern China are investigated.Results indicate that:1)According to the regions of biggest positive SST anomaly during El Nino events,El Nino events are categorized into three patterns,namely,the Nino3,Nino4 and Nino3.4 patterns.2)In the following June of El Nino event year,the significant positive precipitation anomaly mainly locates in the Poyang Lake and Dongting Lake basins for the Nino3 pattern,the Poyang Lake basin,the junction of Guangxi,Guangdong and Hunan,and the western of Guangxi for the Nino4 pattern,and the Dongting Lake basin for the Nino3.4 pattern,respectively.From June to July,the positive precipitation anomaly moves northward into the Yangtze River basin for the Nino3 pattern.The precipitation anomaly shows reverse phase distribution from west(more) to east(less) in August.From June to August,the positive precipitation anomaly gradually moves northward for Nino4 pattern,while does from south to north then northeastward for Nino3.4 pattern.3)In the whole following summer,the significant positive precipitation anomaly of Nino3,Nino4 and Nino3.4 patterns distributes from south to north in eastern China.The positive precipitation anomaly is the strongest for Nino4 pattern and the weakest for Nino3.4 pattern,no matter in June,July,August or summer.4)The different types of El Nino events are corresponding to different circulation characteristics in the following June,July,August or summer.On the whole,the South Asia high becomes stronger and moves eastward and northward corresponding to the climatological field after the events of Nino3 and Nino4 patterns,and it is stronger for Nino3 than that for Nino4.After the event of Nino3.4 pattern,it is weaker and withdraw westward.The western Pacific subtropical high is stronger and moves westward and northward corresponding to the climatological field after the events of Nino3 and Nino4 patterns,and it is stronger moves more westward and northward for Nino3 than that for Nino4.After the event of Nino3.4 pattern,it retreats eastward and moves northward.

Abstract:Based on the WRF ETKF-3DVAR hybrid data assimilation system and 51 members of ECWMF global ensemble prediction in TIGGE data,the flow-dependent characteristics of typhoon forecasting errors,the sensitivity of coupling coefficient in hybrid data assimilation and forecast,and its effects on forecast skill are analyzed,taking typhoon Muifa for example.Results suggest that the forecasting error covariance of typhoon has significant meso-and small-scale characteristics and the structure of forecasting error covariance that is estimated according to ensemble prediction can reappear its flow-dependent nature.The optimal coupling coefficient in hybrid data assimilation scheme can better improve the qualities of analysis and forecast of typhoon than 3DVAR scheme.However,there are obvious effects on typhoon track forecast for different coupling coefficients and an improper coupling coefficient can lead to a worse result.That is,only the relative appropriate flow-dependent information in forecasting error covariance is coupled,the hybrid data assimilation scheme can have positive effects on the qualities of analysis and forecast.It shows that,in the hybrid data assimilation system,constructing a coupling weight function with adaptive ability and achieving automatically the aim to choose optimal coupling coefficient are of importance to improve forecast quality and give full play to potential advantage of hybrid data assimilation system.

Abstract:Based on the FNL reanalysis data and U.S. Joint Typhoon Warning Center(JTWC) tropical cyclone data,the WRF(Weather Research and Forecasting) model is employed to establish the ensemble prediction system for the typhoon Saola(1209) based on the initial perturbation methods of Breeding of Growing Mode(BGM) and Ensemble Transform Kalman Filter(ETKF),respectively.The ensemble forecasting experiments for the typhoon track are conducted.It is detected that the ensemble mean forecasting results using either BGM or ETKF perform much better than control forecasting results in the aspects of wind,temperature and geopotential height fields.And the ETKF method is superior to the BGM method,especially in the aspects of wind and temperature fields.Results show that the dispersions of ensemble forecast system using BGM method are smaller than those using ETKF method.The BGM ensemble forecast system has a fairly limited ability to describe the whole atmosphere.Additionally,the ensemble mean track forecasts based on BGM and ETKF are both superior to the control forecast,and well manage the problem of faster speed after the second landfall in Fujian coast for the control forecast.However,for the landfall location,the predictions by means of the two methods are not so satisfactory.In addition,the ETKF method is considerably better than the BGM method in the ensemble track prediction of typhoon Saola.

Abstract:Based on the Hadley Climtae Center monthly mean sea level pressure(SLP) dataset(SLP2r) from 1850 to 2009,this paper defines and calculates the circulation indices of five atmospheric centers of action(ACAs),including Icelandic low(LIC),North Atlantic subtropical high(HNA),Mongolian high(HMO),Aleutian low(LAL) and North Pacific subtropical high(HNP) in winter season over the Northern Hemisphere,and analyzes the abnormal features of five ACAs and their relationship with climate anomaly in China.The main conclusions are as follows:1)The intensity index(P) of five ACAs in winter and January are positively correlated with the area index(S),so they are not independent.The P and the center position index(λ_c,φ_c) are not completely independent.2)HNA and LAL increase significantly in both160 years(from 1850 to 2009) and recent years(from 1950 to 2009).HMO also increases significantly in 160 years,while LIC enhances significantly in recent years.3)LIC and HNA experience the four similar stages of weak,strong,weak and strong.The P index of HMO can be divided into three stages.HMO is weak during 1850-1910,changes slowly from 1910 to the middle of 1940's,and is strong from the end of 1940's to 2009.LAL is weak from 1856 to 1895 and from 1945 to 1973,and is strong from 1895 to 1945 and from 1973 to 2009.The stage change of HNP is not obvious.4)HMO is the main ACA that affects winter climate and anomaly over China.In the positive phase of HMO,the temperature is lower in China(except the southwestern China),and the precipitation is higher over Sichuan and some areas of North China in winter.LIC and HNA,which locate in the upstream of China,are secondary ACAs that affects on winter climate anomaly in China.When LIC and HNA are stronger,the temperature is higher in some areas of Northeast China,North China and Northwest China.The impacts of LAL and HNP on climate anomaly in China are relatively weak.

Abstract:Using the ozone sonde data derived from GPSO3 ozone sounding system and the NCEP reanalysis data,this paper investigated the persistent ozone peak phenomenon and the abnormal ozone content in the range of 10-14 km height over Beijing in winter 2008 by analyzing the weather situations,atmospheric circulations,upper potential vorticity variations and changes of tropopause height.Results show that,during the special period when there was a snow disaster in South China in 2008,the main reasons of phenomenon of the ozone peak and abnormal content are the strong downward movement of stratospheric air and the stratosphere-troposphere exchange(STE).The ozone downward transport from stratosphere to troposphere is due to the special weather background,including the long time maintenance of the Ural blocking high,the stable existence of the transverse trough between Lake Baikal and Lake Balkhash,and the long-term maintenance of the cut-off low lying on the eastern of the Caspian Sea,leading to the stably southward movement of cold air to affect the vertical ozone distribution over Beijing.Synchronously,with appearance of the subtropical westerly jet,Beijing is located at the left side of the jet entrance areas,and there is a strong convergent and downward movement in upper levels,which is helpful to the air downward transport from stratosphere to troposphere.To sum up,the phenomena of ozone peak and abnormal ozone content shows that the movement of stratospheric air and the STE is much more active over Beijing under the influence of cold air process.

Abstract:Based on the satellite data of moisture and temperature from AURA/MLS,the cloud physical information from CALIPSO/CALIOP,and the reanalysis dataset of ECMWF/Interim,this paper studies the distribution of cloud top above the tropopause(CTAT) events and its effect on water vapor and temperature structures in the upper troposphere and lower stratosphere(UTLS) over East Asia.Results are as follows:1)The maximum frequency of CTAT(reaching 30%-55%) in boreal summer is located in Asian monsoon region.Meanwhile,the maximum frequency of CTAT in the mid-latitudes is located in Northeast Asia,exceeding 15%-20%.2)Based on the CTAT,the composite analysis suggests that there is an anomalous structure of "dry above-moist below" and "cold above-warm below" in UTLS along 15-35°N over East Asia-West Pacific,which is consistent with the composite analysis result based on the tropical cyclones.It means that the tropical cyclones are the main weather systems forming CTAT in this area.3)The composite analysis shows that there is an anomalous structure of "dry above-moist below" and "warm above-cold below" in UTLS along 35-50°N over Northeast Asia,which is consistent with the composite analysis result based on the extratropical cyclones,meaning that the extratropical cyclones dominate the formation of CTAT there.

Abstract:Based on the data from 17 wind masts over Xinjiang wind areas,the aerodynamic roughness lengths extending some ranges are estimated using the standard deviations of wind speed and direction.Then the estimated roughness lengths in the target areas are compared with those defined in numerical model MM5 or WRF or CALMET on account of 3 km and 1 km horizontal resolutions,respectively.Results are as follows:In most of the cases,the defined roughness lengths are moderately consistent with the estimated ones while few of them show major discrepancies due to a misinterpretation of landuse dataset.Thus further emphasis should be on modifying these datasets.The calculated roughness lengths in the strong wind areas are uniformly smaller than the defined roughness in meso-scale model in summer.To attain a better accuracy in cases where the roughness in primary and secondary wind directions is severly nonhomogeneous,the two methods of wind speed and direction standard deviations should be combined.In this study,the roughness lengths are estimated with the samples in neutral atmosphere while they will not be the applicable in areas where neutral atmosphere barely occurs.

Abstract:This paper is aimed to extract the paddy rice area of Jiangsu,Anhui and Jiangxi provinces in 2010 by using multi-temporal MODIS imagery,and to investigate the efficiency of Fast Independent Component Analysis(FastICA) algorithm in dealing with mixed pixels.In order to map the paddy rice area,the time series of Land Surface Water Content Index(I_LSW) and Normalized Difference Vegetation Index(I_NDV) are first produced from MODIS data.Then they are smoothed in order to reduce contamination by clouds.Meanwhile,the pixels with non-crop classes are removed from the index-images using the Land Cover Type data,which is one of the public-distributed MODIS products.The mapping accuracy of the paddy rice area is always affected by the spatial variation of rice planting schedules and rice species.To solve this issue,Rice Similarity Index(RSI) is calculated from I_NDV and used to establish standard rice growing curves for different parts of the study area according to the field survey.Combining the standard rice growing curves and the time series of index-images,a rice mapping algorithm is developed and performed to extract the rice planting area.The results show that the obtained rice distribution map is well consistent with the actual situation.However,the retrieved rice planting area is overestimated in most part of the study area,which can be primarily ascribed to the mixed pixels.In this paper,the FastICA algorithm is adopted to decompose the mixed pixels using time series of I_NDV.Before applying the FastICA algorithm,the images of times series of I_NDV are masked by the obtained paddy rice map.Menwhile,several parameters of the FastICA are optimized by comparisons.As a result,a map of paddy rice abundance fraction is retrieved for each province by FastICA.According to the analysis and comparison with the statistic data,the average accuracies of the obtained rice area are 86.4%,87.9% and 51.5% in Jiangsu,Anhui and Jiangxi provinces,respectively.The larger area error in Jiangxi province can be ascribed to the great proportion of the paddy fields located in the mountainous area.

Abstract:Weather and climate in Beijing area may be affected by surface energy partitioning in Beijing suburbs.To evaluate the performance of land surface models in energy partitioning simulation over sparse grasslands in Beijing suburbs,the original and modified versions of Simple Biosphere Model 2(SiB2) are applied to simulate the surface radiation balance,turbulent energy budget,and soil temperature and moisture over a tank exercise field in Beijing suburban area from 22 July to 5 August 2010.Simulated results are compared against the direct measurements.Results show that:1)The modified SiB2 underestimates net radiation by 5.81% when the original SiB2 underestimates net radiation by 11.32%.The modified SiB2 where the soil thermal conductivity is updated can better simulate the soil temperature(including the soil surface temperature) and the upward-wave radiation;2)The modified SiB2 can better simulate the sensible heat and latent heat fluxes,but both the original and modified SiB2 underestimate the soil heat flux.

Abstract:Based on the meteorological observation data and NCEP/NCAR reanalysis data,this paper analyzes the characteristics and causes of an infrequent snowstorm process accompanied by thunderstorm occurred from 18 to 19 February 2013.Results show that the strong 700 hPa southwest airflow and the obvious 925 hPa eastern jet,which act as the warm cover and the clod pad,provide energetic motivation condition for the heavy snowfall.Favorable upper and lower pumping configuration of divergence is weighted towards the long-duration snowfall.The snowfall is intensified by the riming effect.The snow center corresponds quite well to the concentrated belt of pseudo-equivalent temperature.The thunderstorm that occurrs in cold situation is a typical elevated thunderstorm in this case.The further diagnostic analysis of saturated moist potential vorticity shows that the thunderstorm occurrs in the conditional symmetric instability situation.The isentropic analysis reveals the trigger mechanism of the thunderstorm:The warm airflow climbs along the cold front from low layer to 600-650 hPa and meets the cold air,which triggers the thunderstorm.

Abstract:According to the 3-D hail-category hailstorm model,this paper studies the in-cloud microphysical structures and the hail size distribution characteristics of two multi-cell hail processes occurred in Xining,Qinghai on 20 July and 5 August 1979.Results show that:1)The mixing ratios of cloud water and rainwater present the characteristics of first increasing then decreasing with the development of hail cloud.The hail is mainly formed by frozen undercooling raindrops in the two cases and grows up by deposition and by collecting super cold cloud water.2)The width of hail size distribution is related to updraft.The slantwise updraft is beneficial to hail recirculation growth and leads to wider width of hail size distribution in the former.In the latter,there is not the slantwise updraft,which is not beneficial to hail recirculation growth and leads to narrower width of hail size distribution.

Abstract:Quasi-biweekly oscillation of atmosphere(QBWO) mainly indicates the 10-20 days oscillation on the time scale.It is one of important low-frequency systems of atmosphere and is found when the monsoon weather and its correlative monsoon systems are studied.QBWO can influence the lage scale long-range weather change and its anomaly significantly,which possesses global and multi-seasonal characteristics.This paper systematically summarizes the research results of QBWO in recent years,including the structure and active characteristics of QBWO,its influence on weather and climate and its trigger and maintaining mechanisms.On the basis,some prospects of further researches on QBWO are proposed.